Effects of forest plantations on the genetic composition of conspecific native Aleppo pine populations.

Afforestation is a common and widespread management practice throughout the world, yet its implications for the genetic diversity of native populations are still poorly understood. We examined the effect of Aleppo pine (Pinus halepensis) plantations on the genetic composition of nearby conspecific native populations. We focused on two native populations in Israel with different levels of isolation from the surrounding plantations and compared the genetic diversity of naturally established young trees within the native populations with that of local native adults, using nine nuclear microsatellite markers. We found that the genetic composition of the recruits was significantly different from that of local adults in both populations, with allelic frequency changes between generations that could not be ascribed to random drift, but rather to substantial gene flow from the surrounding planted Aleppo pine populations. The more isolated population experienced a lower gene-flow level (22%) than the less isolated population (49%). The genetic divergence between native populations at the adult-tree stage (F(st) = 0.32) was more than twice as high as that of the young trees naturally established around native adults (F(st) = 0.15). Our findings provide evidence for a rapid genetic homogenization process of native populations following the massive planting efforts in the last decades. These findings have important implications for forest management and nature conservation and constitute a warning sign for the risk of translocation of biota for local biodiversity.

Genetic evidence for a Janzen-Connell recruitment pattern in reproductive offspring of Pinus halepensis trees.

Effective seed dispersal, combining both dispersal and postdispersal (establishment) processes, determines population dynamics and colonization ability in plants. According to the Janzen-Connell (JC) model, high mortality near the mother plant shifts the offspring establishment distribution farther away from the mother plant relative to the seed dispersal distribution. Yet, extending this prediction to the distribution of mature (reproductive) offspring remains a challenge for long-living plants. To address this challenge, we selected an isolated natural Aleppo pine (Pinus halepensis) population in Mt. Pithulim (Israel), which expanded from five ancestor trees in the beginning of the 20th century into ∼2000 trees today. Using nine microsatellite markers, we assigned parents to trees established during the early stages of population expansion. To elucidate the effect of the distance from the mother plant on postdispersal survival, we compared the effective seed dispersal kernel, based on the distribution of mother-offspring distances, with the seed dispersal kernel, based on simulations of a mechanistic wind dispersal model. We found that the mode of the effective dispersal kernel is shifted farther away than the mode of the seed dispersal kernel, reflecting increased survival with increasing distance from the mother plant. The parentage analysis demonstrated a highly skewed reproductive success and a strong directionality in effective dispersal corresponding to the wind regime. We thus provide compelling evidence that JC effects act also on offspring that become reproductive and persist as adults for many decades, a key requirement in assessing the role of postdispersal processes in shaping population and community dynamics.